<?php
$search_str = get_search_query();
?>
<form method="get" id="searchform" action="<?php echo esc_url(home_url('/')); ?>">
    <input type="text" class="field" name="s" id="s" placeholder="<?php esc_attr_e('Search Blog', 'dktm'); ?>" value="<?php echo ($search_str != "") ? esc_attr_e($search_str) : ""; ?>" />
    <input type="hidden" name="post_type" value="post" /> 
</form>